The present invention relates to non-thermoplastic fibers comprising modified starch and processes for making such fibers. The non-thermoplastic starch fibers can be used to make nonwoven webs and other disposable articles.
Natural starch is a readily available and inexpensive material. Therefore, attempts have been made to process natural starch on standard equipment using existing technology known in the plastic industry. However, since natural starch generally has a granular structure, it needs to be xe2x80x9cdestructurizedxe2x80x9d and/or otherwise modified before it can be melt-processed like a thermoplastic material. The task of spinning starch materials to produce fine-diameter starch fibers, or more specifically, the fibers having average equivalent diameters of less than about 20 microns, suitable for production of tissue-grade fibrous webs, such as, for example, those suitable for toilet tissue, presents additional challenges. First, the processable starch composition must possess certain rheological properties that allow one to effectively and economically spin fine-diameter starch fibers. Second, it is highly desirable that the resulting fibrous web, and therefore the fine-diameter starch fibers comprising such a web, possesses a sufficient wet tensile strength, flexibility, stretchability, and water-insolubility for a limited time (of use).
xe2x80x9cThermoplasticxe2x80x9d or xe2x80x9cthermoplastically-processablexe2x80x9d starch compositions, described in several references herein below, may be suited for production of starch fibers having good stretchability and flexibility. The thermoplastic starch, however, does not possess the required wet tensile strength which is a very important quality for such consumer-disposable articles as toilet tissue, paper towel, items of feminine protection, diapers, facial tissue, and the like.
In the absence of strengthening agents, such as, for example, a high level of relatively expensive water-insoluble synthetic polymers, cross-linking may be necessary to obtain a sufficient wet tensile strength of starch fibers. At the same time, chemical or enzymatic agents have been typically used to modify or destructurize the starch to produce a thermoplastic starch composition. For example, a mix of starch and a plasticizer can be heated to a temperature sufficient to soften the resulting thermoplastic starch-plasticizer mix. In some instances pressure can be used to facilitate softening of the thermoplastic mix. Melting and disordering of the molecular structure of the starch granule takes place and a destructurized starch is obtained. However, the presence of plasticizers in the starch mix interferes with cross-linking of the starch and thus discourages the resulting starch fibers from acquiring a sufficient wet tensile strength.
Thermoplastic or thermoplastically-processable starch compositions are described in several US patents, for example: U.S. Pat. Nos. 5,280,055 issued Jan. 18, 1994; U.S. Pat. No. 5,314,934 issued May 24, 1994; U.S. Pat. No 5,362,777 issued November 1994; U.S. Pat. No. 5,844,023 issued December 1998; U.S. Pat. No. 6,117,925 issued Sep. 12, 2000; U.S. Pat. No. 6,214,907 issued Apr. 10, 2001; and U.S. Pat. No. 6,242,102 issued Jun. 5, 2001, all seven immediately preceding patents issued to Tomka; U.S. Pat. No. 6,096,809 issued Aug. 1, 2000; U.S. Pat. No. 6,218,321 issued Apr. 17, 2001; U.S. Pat. Nos. 6,235,815 and 6,235,816 issued on May 22, 2001, all four immediately preceding patents issued to Lorcks et al.; U.S. Pat. No. 6,231,970 issued May 15, 2001 to Andersen et al. Generally, the thermoplastic starch composition can be manufactured by mixing starch with an additive (such as a plasticizer), preferably without the presence of water as described, for example, in U.S. Pat. No. 5,362,777 referenced herein above.
For example, U.S. Pat. Nos. 5,516,815 and 5,316,578 to Buehler et al. relate to thermoplastic starch compositions for making starch fibers from a melt-spinning process. The melted thermoplastic starch composition is extruded through a spinneret to produce filaments having diameters slightly enlarged relative to the diameter of the die orifices on the spinneret (i.e., a die swell effect). The filaments are subsequently drawn down mechanically or thermomechanically by a drawing unit to reduce the fiber diameter. The major disadvantage of the starch composition of Buehler et al. is that it requires significant amounts of water-soluble plasticizers which interfere with cross-linking reactions to generate apparent peak wet tensile stress in starch fibers.
Other thermoplastically processable starch compositions are disclosed in U.S. Pat. No. 4,900,361, issued on Aug. 8, 1989 to Sachetto et al.; U.S. Pat. No. 5,095,054, issued on Mar. 10, 1992 to Lay et al.; U.S. Pat. No. 5,736,586, issued on Apr. 7, 1998 to Bastioli et al.; and PCT publication WO 98/40434 filed by Hanna et al. published Mar. 14, 1997.
Some of the previous attempts to produce starch fibers relate principally to wet-spinning processes. For example, a starch/solvent colloidal suspension can be extruded from a spinneret into a coagulating bath. References for wet-spinning starch fibers include U.S. Pat. No. 4,139,699 issued to Hernandez et al. on Feb. 13, 1979; U.S. Pat. No. 4,853,168 issued to Eden et al. on Aug. 1, 1989; and U.S. Pat. No. 4,234,480 issued to Hernandez et al. on Jan. 6, 1981. JP 08-260,250 describes modified starch fibers manufactured from starch and an amino resin precondensate, and a method for making the same. The method includes dry spinning of an undiluted solution of starch and amino resin precondensate, followed by heat treatment. The starch used in this application is natural starch, such as contained in corn, wheat, rice, potatoes etc.
The natural starch has a high weight average molecular weightxe2x80x94from 30,000,000 grams per mole (g/mol) to over 100,000,000 g/mol. The melt-rheological properties of an aqueous solution comprising such starch are ill-suited for high-speed spinning processes, such as spun-bonding or melt-blowing, for production of fine-diameter starch fibers.
The art shows a need for an inexpensive and melt-processable starch composition that would allow one to produce fine-diameter starch fibers possessing good wet tensile strength properties and suitable for production of fibrous webs, particularly tissue-grade fibrous webs. Consequently, the present invention provides non-thermoplastic fine-diameter starch fibers having sufficient apparent peak wet tensile stress. The present invention further provides a process for making such non-thermoplastic starch fibers.
The invention comprises a process for making non-thermoplastic starch fibers that have no melting point. In one aspect, the process comprises the steps of: (a) providing a non-thermoplastic starch composition comprising from about 50% to about 75% by weight of modified starch and from about 25% to about 50% of water and having a shear viscosity from about 1 to about 80 Pascalxc2x7seconds (Paxc2x7s) at the processing temperature and at a shear rate of 3,000 secxe2x88x921; (b) extruding the non-thermoplastic starch composition through a plurality of extrusion nozzles, each terminating with a nozzle tip, thereby forming a plurality of embryonic starch fibers; (c) attenuating the plurality of embryonic starch fibers with an attenuating air having an average velocity at the nozzle tips greater than about 30 meters per second, to cause the fibers to form individual average equivalent diameters of less than about 20 microns; (d) dewatering the embryonic starch fibers to a consistency of from about 70% to about 99% by weight, thereby producing non-thermoplastic starch fibers having no melting point. The process may further comprise a step of humidifying the attenuating air so that the attenuating air has a relative humidity at the nozzle tips greater than about 50%. The step of dewatering the embryonic fibers may comprise drying the embryonic fibers with a drying air having a temperature from about 150xc2x0 C. to about 480xc2x0 C., and more specifically from about 200xc2x0 C. to about 320xc2x0 C., and relative humidity of less than about 10%.
In another aspect the invention comprises a process for making non-thermoplastic starch fibers, comprising the steps of: (a) providing a non-thermoplastic starch composition as described above; (b) extruding the non-thermoplastic starch composition through a plurality of extrusion nozzles, each terminating with a nozzle tip, thereby forming a plurality of embryonic starch fibers, wherein the plurality of nozzles are arranged in multiple rows to form an attenuation zone extending from the nozzle tips to an attenuation distance in the direction of the starch composition flow; (c) providing an attenuating air having a relative humidity greater than about 50% at the nozzle tips; (d) attenuating the plurality of embryonic fibers with the attenuating air having a velocity greater than about 30 meters per second at the nozzle tips, thereby producing a plurality of non-thermoplastic starch fibers having individual average equivalent diameters of less than about 20 microns; and (e) dewatering the non-thermoplastic starch fibers to a consistency of from about 70% to 99% by weight.
The non-thermoplastic starch composition may comprise from about 0.1% to about 10% by weight of a cross-linking agent. The non-thermoplastic starch composition may further comprise from about 0.1% to about 15% by weight of a polycationic compound selected from the group consisting of divalent or trivalent metal ion salt, natural polycationic polymers, synthetic polycationic polymers, and any combination thereof.
The process may further comprise a step of maintaining the relative humidity in the attenuation zone greater than about 50%, and more specifically greater than about 60%. The step maintaining the relative humidity in the attenuation zone may comprise a step of providing a physical enclosure of the attenuation zone, or alternatively or additionally, a boundary air around the attenuation zone. The boundary air can be supplied through a plurality of discrete orifices arranged to surround the attenuation zone. Alternatively, the boundary air can be supplied through continuous slots arranged to surround the attenuation zone. The plurality of discrete orifices or continuous slots can be structured to provide the boundary air having a velocity substantially equal to the velocity of the attenuating air. The boundary air can beneficially have a relative humidity of from about 50% to about 100%.
The process may further comprise a step of providing a secondary attenuating air downstream the attenuating air, through at least one secondary attenuating air nozzle. The secondary attenuating air may have a velocity of from about 30 meters per second (m/sec) to about 500 m/sec, and more specifically from about 50 m/sec to about 350 m/sec, as measured at a secondary attenuating air nozzle exit. The secondary attenuating air may have a temperature from about 20xc2x0 C. to about 480xc2x0 C., and more specifically from about 70xc2x0 C. to about 320xc2x0 C.
For the purposes of making a fibrous web or other consumer products, the process can further comprise a step of collecting the non-thermoplastic starch fibers on a working surface, such as, for example, a foraminous belt.
In another aspect, the invention comprises a process for making non-thermoplastic starch fibers, comprising the steps of: (a) providing a non-thermoplastic starch composition having the shear viscosity described above and comprising from about 50% to about 75% by weight of modified starch, from about 25% to about 50% of water, from about 0.1% to about 10% by weight of a cross-linking agent, and from about 0.1% to about 15% by weight of a polycationic compound selected from the group consisting of divalent or trivalent metal ion salt, natural polycationic polymers, synthetic polycationic polymers, and any combination thereof; (b) extruding the non-thermoplastic starch composition through a plurality of extrusion nozzles arranged in multiple rows and forming an attenuation zone extending to an attenuation distance from tips of the nozzles in the direction of the starch composition flow, thereby forming a plurality of embryonic starch fibers; (c) attenuating the plurality of embryonic fibers with an attenuating air having a relative humidity greater than about 50% and a velocity at the extrusion nozzle tips from about 30 m/sec to about 500 m/sec; (d) further attenuating the plurality of embryonic fibers with a secondary attenuating air downstream the attenuating air to form a plurality of non-thermoplastic fibers having individual average equivalent diameters less than about 10 microns; and (e) dewatering the plurality of embryonic non-thermoplastic starch fibers to a consistency from about 70% to about 99% by weight, thereby forming a plurality of non-thermoplastic starch fibers having no melting point. The process may further comprise a step of providing at least a partial boundary layer of a humidified air around the attenuation zone.
In still another aspect, the invention comprises a process for making non-thermoplastic starch fibers, comprising steps of: (a) providing a non-thermoplastic starch composition; (b) extruding the non-thermoplastic starch composition through at least one extrusion nozzle terminating with a nozzle tip, thereby forming at least one embryonic starch fiber; (c) attenuating the at least one embryonic starch fiber with an attenuating air having an average velocity at the nozzle tip greater than about 30 meters per second, to cause the fiber to form an average equivalent diameter of less than about 20 microns; and (d) dewatering the at least one embryonic starch fiber to cross-link the starch in the fiber so that the fiber has a salt-solution absorption capacity less than about 2 grams of a salt solution per 1 gram of fiber, and more specifically less than about 1 gram of a salt solution per 1 gram of fiber. In the step of providing a non-thermoplastic starch composition, the non-thermoplastic starch composition may have a pH from about 1.5 to about 5.0, more specifically from about 2.0 to about 3.0, and still more specifically from about 2.2 to about 2.6.